Boolean Expressions

Hello there! I'm Saavi, and I'm so glad you're here. Today, we're diving into one of the most important concepts in all of programming: making decisions.

At its heart, a computer program is often just following a list of instructions. But the real power comes when a program can make a choice. To do that, it first needs to ask a question.

The Question and the Answer: Boolean Expressions

In Java, a question that can be answered with a simple true or false is called a Boolean expression.

Think about these questions:

• Is the temperature outside less than 32°F? (true or false)
• Is your grade in this class greater than or equal to 90? (true or false)
• Is your name "Marcus"? (true or false)

Every single one of these evaluates to a simple, two-option answer. This true or false value is a fundamental data type in Java, called a boolean.

An expression that results in a boolean value is a Boolean expression. This directly covers one of our key learning goals: any expression using the operators we're about to discuss will evaluate to a boolean.

The Tools for Asking: Relational Operators

To build these questions in Java, we use relational operators. They let us compare two values.

Let's look at the operators for numbers (int, double, etc.):

[[fig:greater_than_or_equal_mistake]]

int myScore = 88;
int passingScore = 70;

boolean isHigher = myScore > passingScore; // 88 > 70 is true
boolean isPerfect = myScore >= 100;      // 88 >= 100 is false

System.out.println(isHigher);  // Prints: true
System.out.println(isPerfect); // Prints: false

Checking for Sameness: == and !=

What if we want to know if two values are exactly the same or not? We have two more operators for that.

• int x = 5; means "put the value 5 into the variable x."
• x == 5; means "is the value in x the same as 5?" (This would evaluate to true).

If you use a single = when you mean to compare, you'll get an error. It's a rite of passage for every new programmer to make this mistake. Just be aware of it!

The Big Trap: Primitives vs. Objects

Now for the most important, and trickiest, part of this lesson. The == operator behaves differently depending on what you are comparing.

Think of it like this: imagine you have two friends, Liam and Sofia, and you give them each a $5 bill.

• A primitive type (int, double, boolean) is like the value on the bill. If you ask, "Does Liam's bill have the same value as Sofia's bill?", the answer is yes. They're both worth $5.
• When you use == with primitives, you are comparing their values.

int liamsValue = 5;
int sofiasValue = 5;
boolean areValuesEqual = (liamsValue == sofiasValue); // This is true. 5 is equal to 5.

Now, imagine you have one car—a specific, single Toyota Camry parked in a specific spot in a Chicago parking garage.

• An object (like a String or any other non-primitive type) is like that specific car.
• A reference variable is like a slip of paper with the car's location written on it ("Section A, Spot 32").

If you give Liam and Sofia each a slip of paper with "Section A, Spot 32" written on it, they are both pointing to the exact same car.

If you buy a second, identical Toyota Camry and park it in "Section B, Spot 18," and give Sofia a new slip of paper with that location, they now have two different cars that just happen to look the same.

When you use == with objects, you are asking: "Do these two slips of paper point to the exact same spot in the parking garage?" You are not asking if the cars are identical.

// Scenario 1: Two references, ONE object.
String carLocation = new String("Section A, Spot 32");
String liamsSlip = carLocation;
String sofiasSlip = carLocation;

// Are they pointing to the SAME object in memory?
boolean sameObject = (liamsSlip == sofiasSlip); // This is true.

// Scenario 2: Two references, TWO different but identical objects.
String car1 = new String("Toyota Camry");
String car2 = new String("Toyota Camry");

// Are they pointing to the SAME object in memory?
boolean sameCarObject = (car1 == car2); // This is FALSE!

This is a huge source of bugs. car1 and car2 are two separate objects in your computer's memory that happen to contain the same text. The == operator sees two different memory addresses and returns false. We'll learn the correct way to compare the contents of objects (using a method called .equals()) in a later unit, but for now, you must understand that == on objects checks for an identical location in memory, not identical content.

Let's walk through a few examples to make this crystal clear. The key is to read the expression, substitute the values, and then evaluate to true or false.

Time to try it on your own. Don't worry about writing a full program, just focus on writing the single line that represents the Boolean expression.

1. 1
   Social Media Verification

   You're coding a feature for a new app based in Seattle. To get a "power user" badge, a user must have more than 500 posts. Create a boolean variable isPowerUser and assign it the result of an expression that checks if a user with numPosts qualifies.
2. 2
   Sales Tax Exemption

   In some states, certain items are exempt from sales tax. Let's say an item is tax-exempt if its category code is exactly 107. Write a Boolean expression that evaluates to true if itemCode is tax-exempt.